The present invention relates to a process for the selective synthesis of silylalkyldisulphides by the desulphurization of corresponding polysulphides with nucleophilic reagents.
Trialkoxypropylpolysulphides are excellent phase mediators for the incorporation of oxide materials into rubber matrices. In particular in the tire industry, triethoxysilylpropyltetrasulphane ([(CH.sub.3 CH.sub.2 O).sub.3 SiCH.sub.2 CH.sub.2 CH.sub.2 ].sub.2 S.sub.4) is widely used in silica-reinforced tires. In such applications, the silane, on the one hand, becomes attached to free hydroxyl groups of the silica and, on the other, undergoes vulcanization-like crosslinking with the rubber. In specific applications, it is convenient to provide the silane not with a tetrasulphane functional group, but instead with a less reactive disulphane functional group. The synthesis of silylalkyldisulphanes together with the corresponding polysulphides is described in various patents and publications.
German Patents 2 405758 and 2 542534 relate to production starting from mercaptoalkylsilanes and sulphur, in which hydrogen sulphide is released.
Various production processes start from disulphides produced in situ, with which nucleophilic substitutions are then performed on haloalkylsilanes. These processes differ only in the synthesis of the disulphide nucleophile. According to German Patent 3 311340, the disulphide is produced by reactions between hydrogen sulphide, sodium and sulphur in ethanol.
According to U.S. Pat. No. 5,405,985, preparation is performed using an aqueous sodium sulphide solution together with sulphur. It is sufficiently well know to those skilled in the art that mixtures of various polysulphides are produced in reactions between sulphides and sulphur, such that when nucleophilic substitution is performed it is only possible to produce a mixture of polysulphanes of various chain lengths. The same applies to reactions between mercaptans or thiolates and sulphur. It is moreover known that the corresponding disulphanes may be isolated from these product mixtures only with great difficulty.
While German Patent 2 360470 does indeed describe a method for the production of pure bis(silylalkyl)disulphane by oxidizing the corresponding mercaptan with sulphuryl chloride, this method results in the formation of highly corrosive secondary products (SO.sub.2, HCl). Secondary reactions on the silyl residue moreover result in a reduction in the yield of the desired product (for example: 63.3%). Another oxidative variant is described in EP-A1 217178. In this variant, the corresponding thiolates are oxidized with iodine to yield the disulphides. After the elaborate production of the silylalkylmercaptan, this process requires two further reaction stages.
It is therefore an object of the present invention to obtain elevated yields of the desired silylalkyldisulphides.